Spontaneous and induced chromosome and nuclear abnormalities are being studied in cancer cells and in cells transformed in vitro. (1) An attempt is being made to study the viability of cell populations in which chromosome pulverization has been induced. If viability is reduced this would be evidence for actual damage to chromosomes when pulverization is observed. (2) Enhanced secondary constrictions are common in cancer cells. Benzamide has been observed to enhance the incidence of spontaneous constrictions in cell lines. The effect of benzamide on virus-induced constrictions will be studied. (3) Further studies will be done on nuclear division control mechanisms in cytochalasin-B treated normal, transformed, and cancer cells. Addition of proteases may cause normal cells to act like malignant cells when treated with cytochalasin. (4) "Complex Chromosome Abnormalities" consist of multiple breaks, exchanges, etc., observed usually only in transformed and cancer cells. Studies are being done on TLCK, a protease inhibitor, which appears to increase the incidence of complex chromosome abnormalities. (5) RPMI 2650 is a human cell line which has a high frequency of cells with multiple secondary constrictions. Attempts are being made to fuse these cells with Lesch-Nyan fibroblasts to determine if such hybrids show the secondary constrictions in the chromosomes both of 2650 and Lesch-Nyan cells.